Ghoul Guys are gray Boo Guys and appears in Mario & Luigi: Partners in Time. Like the Boo Guy, they live in Shroob Castle, and attack in the same manner (albeit at an increased speed). They now have the ability to possess other enemies and increase their stats. In battle, a Ghoul Guy will roll into either Mario or Luigi — depending on who you're playing as — inflicting damage. However, preceding this, it will split into two transparent duplicates; one of which is the real Ghoul Guy, and the other one is just an impersonator. Your methods of disambiguation are quite simple; only the real Ghoul Guy can injure the Mario Bros.. The Ghoul Guy will point to the Mario brother the ghost will attack.

The difference between a Ghoul Guy and a Boo Guy is that a Ghoul Guy can merge its spirit with another enemy in the background. While the Ghoul Guy will disappear, the other creature will inherit the Ghoul Guy's stats, add them to its own, and will continue with the combined stats. For this reason, ensure that the Ghoul Guys are eliminated from existence before dealing with anything else.